Optical dispersion, that is, the separation of light into its constituent wavelength components, is a phenomenon used by a wide variety of applications, including Raman and fluorescence detection and other forms of spectral analysis. In addition, the emerging field of optical communications uses optical dispersion to perform wavelength multiplexing and demultiplexing, filtering and other functions. Although the concept of optical dispersion has been known for quite some time, the earliest apparatus utilized prisms as a diffraction means. Optical gratings were later developed for this purpose, and, since the invention of holography, holographic gratings have been applied to this task with enhanced efficacy.
It is known to pass polychromatic light through a pair of identical gratings that act together to provide an output beam which is both collimated and laterally dispersed. Such an arrangement is disclosed by E. B. Treacy in "Optical Pulse Compression With Diffraction Gratings," IEEE Journal of Quantum Electronics, Vol. QE-5, No. 9, Sep., 1969, which finds particular application in pulse compression for ultrafast laser systems that employ chirped-pulse amplification. The first grating diffracts each wavelength through a different angle according to the grating equation, thereby introducing angular dispersion to the polychromatic beam, so that the beam spreads as it propagates from the first grating toward the second. The second grating diffracts each wavelength again through the same angle, but in the opposite direction, so that the beam leaves the second grating in the same direction as the beam that was incident to the first grating, with the various wavelengths being spread laterally but propagating in exactly the same direction, or recollimated. One disadvantage of this configuration is that the gratings and auxiliary optics are separate elements that must be individually mounted and aligned, with the attendant risk of alignment drift with time or mechanical motions such as vibration.
It is also known that dispersion may be increased by passing light through a plurality of gratings, each grating further dispersing the light incident to it. In "Double dispersion from dichromated gelatin volume transmission gratings," Proceedings of the SPIE, vol 1461, 1991, D. E. Sheat, G. R. Chamberlin, and D. J. McCartney disclosed a configuration wherein light is passed through a single grating two times with the aid of a mirror, either separate from the grating or made part of the grating to form an integrated device. However, this configuration is limited to two passes of the light through the grating, and the beam that exits from the integrated device is counter-propagating with respect to the incident beam, so that separating the input and output beams requires additional optical components or performance compromises. Moreover, the configuration described by Sheat, et. al. only produces angularly dispersed light, so that conversion to a laterally dispersed, collimated beam again requires additional optical components.
There are also described in the literature dispersive optical elements specifically intended for optical communications. Such a structure is described by Y. Huang, D. Su, and Y Tsai in "Wavelength-division-multiplexing and--demultiplexing by using a substrate-mode grating pair," Optics Letters, Vol. 17, No. 22, Nov. 15 1992. According to this device, within a substrate-mode element there are two distinct gratings which first angularly disperse and then recollimate incident light. The output channel separation or the spatial dispersion of such a structure is directly related to the angular dispersion obtained through the first grating and the distance the dispersed light travels before being collimated by the second grating. The amount of dispersion in the substrate-mode element is therefore dependent on the length as well as the thickness of the substrate.
In a practical sense, the substrate must therefore be long to provide substantial optical distance between the dispersing grating and the collimating grating to obtain high degree of spatial dispersion. Additionally, the space between the dispersing and collimating grating cannot include a grating, or the total internal reflection necessary for propagation would be prevented.
Another prior-art device is described by R. Kostuk, et. al. in "Reducing alignment and chromatic sensitivity of holographic optical interconnects with substrate-mode holograms," Applied Optics, Vol. 28, No. 22, Nov. 15, 1989. The structure of the substrate-mode element described in this paper incorporates a holographic grating as an input element to produce a +1 and a +1 diffracted order from the incident light. These orders propagate through the substrate by means of multiple internal reflections until intercepted by holographic optical elements which redirect, focus, and couple each beam out of the structure and onto receivers. The purpose of this structure is to produce multiple beams output into some preferred spatial arrangement from a single incident beam of coherent light.
There continues to exist, therefore, an outstanding need for an optically dispersive structure which may take advantage of the same grating to achieve a multiplicative dispersive effect, ideally, to achieve a high degree of direct lateral dispersion from a monolithic component.